The present invention relates to a method for manufacturing a semiconductor elemental device wherein a device isolation layer is formed in an SOI layer of an SOI (Silicon On Insulator) substrate by a LOCOS (Local Oxidation of Silicon) method.
An SOI substrate is formed by sequentially laminating a support substrate, an insulating layer (so-called BOX oxide film) and a silicon thin film layer (called SOI layer) formed of monocrystalline silicon. Since a field oxide film electrically insulates and separates between elements of a semiconductor elemental device formed in an SOI layer of the SOI substrate to make it possible to suppress an soft error and latchup and reduce a junction capacitance of a source/drain section, such an SOI substrate has been used in the manufacture of a number of semiconductor devices as a technique that can contribute to speeding up and a reduction in power consumption.
A trench structure and a LOCOS method are used to form the field oxide film that separates between the elements. However, the trench structure has a disadvantage that since an SOI layer is etched to define trenches and an oxide film is embedded in the trenches, the number of processes increases and the cost of manufacturing thereof increases as compared with the LOCOS method.
On the other hand, the LOCOS method is advantageous to a reduction in manufacturing cost. It is however known that a thin silicon layer (called an edge portion) having a triangular section is formed at a boundary among an insulating layer, a field oxide film and an SOI layer upon formation of the field oxide film by the LOCOS method and constitutes a parasitic MOSFET (MOS (Metal Oxide Semiconductor) Field Effect Transistor), and this parasitic MOSFET exerts an adverse effect on current characteristics of an N channel MOS element (called an nMOS element) and is brought to a bump characteristic in which a bump occurs in current-voltage characteristics, thereby reducing a threshold voltage.
As the technique of preventing such a reduction in the threshold voltage, a simulation result has been reported that the bump characteristic can be suppressed by raising a boron concentration of the edge portion formed in the SOI layer (refer to, for example, a non-patent document 1 (J. W. Thomas and two more ones, “Characteristics of Submicrometer LOCOS Isolation”, Proceedings 1995 IEEE International SOI Conference, IEEE, October 1995, p. 116-117)).
The inventors have actually prototyped an nMOS element using conditions shown in Table 1 of the non-patent document 1 and evaluated its current-voltage characteristics.
Process steps at this time are as follows: A pad oxide film is formed in its corresponding SOI layer of an SOI substrate. A silicon nitride film is deposited on the pad oxide film by a CVD (Chemical Vapor Deposition) method and thereafter patterned by photolithography and etching, thereby removing the pad oxide film and silicon nitride film in a device isolation region and forming a filed oxide film in the exposed SOI layer by a LOCOS method.
After the formation of the field oxide film, boron ions are implanted to raise the boron concentration of each edge portion, and thereafter the pad oxide film and silicon nitride film in the corresponding transistor forming region are removed.
Thereafter, the formation of a gate oxide film, the formation of a gate and the formation of a source/drain section are executed in a manner similar to the normal nMOS element manufacturing process, and the corresponding nMOS element used for evaluation was formed.
The result of measurement of current-voltage characteristics of the nMOS element prototyped in this way is shown in FIG. 10.
FIG. 10 shows a drain current per unit width, which flows between the source and drain with respect to each gate voltage. It is understood from the current-voltage characteristics of the nMOS element prototyped according to the above that as shown in FIG. 10, a bump occurs in a region enclosed with a circle and there is room for improvement in bump characteristic.
For the purpose of improving such a bump characteristic, the inventors have proposed that after the formation of a field oxide film, P type impurity ions such as boron are implanted in an SOI layer to form a high-concentration impurity region at an edge portion, and on such a heat-treatment condition that the impurity in the high-concentration impurity region is not diffused into a channel section before the removal of a pad oxide film and a silicon nitride film in a transistor forming region, the damage of the field oxide film due to ion implantation at the formation of the high-concentration impurity region is recovered and the amount of cutting of the field oxide film is reduced (refer to, for example, a patent document 1 (Japanese Unexamined Patent Publication No. 2003-124303 (paragraph numbers 0019-0021 in page 4, FIG. 2 and FIG. 3)).
It has been proposed by the specification disclosed in Japanese Patent Application No. 2003-328092 that a metal oxide film such as aluminum oxide is formed in a slope or inclined part of an edge portion or a device isolation region of an SOI layer before a field oxide film is formed in the SOI layer having exposed the device isolation region by a LOCOS method, and the field oxide film is formed on the metal oxide film to thereby reduce a bump characteristic through the use of a negative fixed electrical charge contained in a defect produced due to the reaction of an interface between the SOI layer and the metal oxide film.
However, the technique of the non-patent document 1 referred to above is accompanied by a problem that although the simulation result that the bump characteristic can be suppressed by enhancing the boron concentration of the edge portion formed in the SOI layer is obtained, the above-described bump characteristic appears in an actual nMOS element.
That is, the simulation result that the bump characteristic can be suppressed by enhancing the impurity concentration of boron or the like at the edge portion, is considered to be correct. However, it is considered to be proper because the impurity of the edge portion is diffused due to heat treatment executed in the subsequent process and eventually the impurity concentration of the edge portion is reduced so that the effect of suppressing the bump characteristic is degraded. This becomes a problem particularly important for an increase in the thermal processing process with multifunctioning of the recent semiconductor device.